This invention relates to a pulp washing apparatus of the type including a vacuum drum rotating horizontally about its longitudinal axis. Such drums are perforated and partially immersed in a fluid medium defined by paper pulp. As one portion of the drum continually rotates out of the pulp, pulp is adhered to the drum exterior surface by the vacuum in the drum interior. This vacuum also functions to draw some of the water from the pulp. At a downstream angular location on the drum surface, a doctor blade is positioned to scrape off the partially dried or partially de-watered pulp mat.
Conventional vacuum drum filters are designed to wash pulp by displacing the fluid carrier (water) through the pulp mat with cleaner water and to discharge the pulp at uniform consistency. The driving force may be a vacuum created by a gravity induced drop leg from the drum filter to a service tank approximately 30 feet below. As the water is pulled into the drum deck by vacuum, a significant amount of air (about 50% of total hydraulic flow) is introduced into the washer system. Reducing the air volume passing through the pulp mat increases the vacuum and can improve the water removal potential of a particular washer. This results in improved efficiency for an overloaded washer or increased capacity for one that is not.
There are inherent variations in displacement efficiency and mat dryness across the width of any conventional vacuum drum filter. This is due mainly to the unevenness of the pulp mat profile, i.e., its variable thickness. Poor inlet vat delivery, uneven shower flow across the drum face, and unbalanced shower flows are typical factors that can affect the mat profile and uniformity. Any improvement in mat uniformity would make the recovered pulp cleaner, this being a primary goal of the drum filter.
It is already known to compress the pulp cake or mat on the exterior surface of a vacuum drum, as shown for example in U.S. Pat. No. 4,737,277 issued to Lenac Apr. 12, 1988, hereby incorporated by reference. However, the Lenac vacuum expression belt 24 does not supply wash water to the pulp mat or cake. His belt 24 merely squeezes or compresses the mat. Further, in such an arrangement, the belt contacts and squeezes the pulp cake or mat on the vacuum drum over an appreciable angular extent and thus the nip pressure is limited to the weight of his endless belt arrangement supported over a relatively large area.